The present invention relates generally to apparatus for continuously measuring fluid density, and more particularly to apparatus for compensating a fluid metering system for variations in fluid density.
Aircraft engine fuel control systems have established and controlled volumetric fuel flow rates, but the mass flow rate is usually required to properly meter and control heat release in the engine combustors. The conversion of volumetric flow rate to mass flow rate in the control system has been predicated on the known density for a given fuel batch. It is well recognized, however, that the known density varies substantially in flight due to variations in fuel temperature. Fuel control systems have compensated for this variation by sensing the temperature and applying a corresponding correction, for example, to the reference setting of the volumetric metering valve. The temperature compensation is often accomplished with a bi-metallic spring but the compensation accuracy, with wide variations in fuel temperature, is limited by the linearity and repeatability of the temperature-to-spring force conversions. Furthermore, prior art systems offer no automatic compensation for density variations which occur between batches of the various hydrocarbon fuel types. Manual adjustment for these basic density variations among fuel types is typically provided in the metering control systems, and the proper reference setting is made by the crew prior to flight based on the fuel type used.